Etching processes are used for selectively removing metal and dielectric films in the fabrication of optical and electronic devices. These etching processes usually exhibit either anisotropic or isotropic etching behavior. Anisotropic etching involves etching along only one crystal axis of the material or along each of the crystal axes of the material at substantially different rates. This type of etching typically produces vertical rather than lateral etching. Dry etching processes such as ion beam milling, reactive ion etching and plasma etching, generally exhibit anisotropic etching behavior.
Isotropic etching behavior involves etching rates which are generally the same along plural crystal axes of the material. Isotropic etching typically produces both vertical and lateral etching of material. The lateral etching usually undercuts the mask layer. Wet chemical etching generally exhibits isotropic etching behavior, and some dry etching processes can also exhibit isotropic etching.
Gauging the amount of undercutting during isotropic etching is important for maintaining repeatability and consistency of device fabrication processes. The amount of undercutting can be determined visually in some cases if the material being etched involves a single layer or multiple transparent layers. However, where the material involves opaque multiple layers, the undercutting of the lowest layer or layers may be visually hidden by the layers on top. This inability to determine undercut is especially troublesome when the etchant is selective to the lower hidden layer, i.e., where the hidden layer etches at a significantly faster rate than the overlying layer.
Conventional methods such as cross-section SEM, TEM and additional selective etching have been used for gauging the amount of undercutting of visually hidden layers. Unfortunately, all of these methods involve costly destruction of the device.
Accordingly, a non-destructive method for determining the amount of undercutting of visually hidden layers is needed for maintaining repeatability and consistency in isotropic etching processes used in optical and electronic device fabrication.